1. Field of the Invention
The present invention relates to an image forming apparatus in which a developer image borne by a belt is transferred to a recording material, particularly to the image forming apparatus in which a developer hardly adheres to a bending member when the bending member abuts on a developer image bearing surface side of the belt.
2. Description of the Related Art
Recently, in a copying machine and a printer in which electrophotographic technology is utilized, a configuration in which the belt conveying a developer image is used is frequently adopted because the configuration has advantages such as a high degree of freedom of arrangement and high flexibility to various media.
For the configuration in which the high degree of freedom of arrangement is utilized, the configuration in which a roller is arranged inside the belt and the bending member including another roller presses a surface side of the belt is increasingly adopted.
The use of the bending member can decrease an excessive space inside the belt as much as possible, and thereby the space of the whole apparatus body can be decreased.
Because the bending member abuts on the developer transfer surface side of the belt, a developer is easy to adhere to the bending member. Therefore, for example, Japanese Patent Application Laid-Open No. 8-146706 proposes that the developer is prevented from adhering to the bending member by applying a bias having the same polarity as the polarity of the developer on the belt to the bending member. However, in applying the voltage having the same polarity as the polarity of the developer to the bending member, the voltage applied to the bending member differs from the transfer voltage in polarity, when a transfer voltage having the polarity opposite to the developer from a recording material backside in a transfer portion. Therefore, a potential difference is increased between the bending member and the transfer portion, which results in a risk of generating electrical interference depending on a resistance of an intermediate transfer member.
For example, in the case where the image is formed using a negatively charged developer, the transfer voltage having the positive polarity opposite to the developer is applied from a recording medium backside to a first transfer member, and the transfer is performed by passing a current between the first transfer member and the grounded second transfer member. The developer is prevented from adhering to the bending member by applying the voltage having the same negative polarity as the developer to the bending member which bends the belt.
At this point, the large potential difference between the transfer portion and the bending member causes a part of the transfer current to be easily passed in the bending member direction, which relatively decreases the current passed in the first transfer member direction. Experience shows that the transfer current strays from an optimum value to cause defective transfer unless the decrease in transfer current is suppressed to 5% or less, preferably to 3% or less.